(1) Field of the Invention
The present invention relates to a digital communication network and, in particular, to a monitoring system for monitoring a protection channel provided for protecting transmission channels from fault in the digital communication system.
(2) Description of the Prior Art
A digital communication network usually has one or more protection or stand-by channels in addition to regular or normally active channels. When any failure such as bit error, out-of-frame synchronization or others occurs in any one of the regular channels, the faulty regular channel is switched over the protection channel so that the data transmission over to the channel is continued without failure. References are made to, for example, U.S. Pat. No. 4,442,518 by Morimoto assigned to Nippon Electric Company, Ltd. and U.S. Pat. No. 4,477,895 by Casper et al assigned to Harris Corporation.
In order to monitor condition of the protection channel, the digital communication network has a protection channel monitoring system.
In a known protection channel monitoring system disclosed in Japanese patent application laid open with No. 60-214135, a check signal is encoded to an AMI (Alternate-Mark-Inversion) code signal. The AMI code signal is split delivered to a plurality of transmission lines or transmission signals which are transmitted to a receiver through a protection channel to be monitored. In an actual manner, of a special example, the AMI code is delivered to a plurality of modulators wherein a carrier wave is modulated by the split AMI code signals according to a plurality of different modulating methods to obtain a plurality of different modulated waves. The modulated waves are transmitted to the receiver through the protection channel.
The receiver receives the respective modulated waves and detects the split AMI code signals from the modulated waves. The split AMI code signals are decoded to NRZ (Non-Return-to-Zero) code signals by the sampling technique at AMI code decoding circuits, respectively.
A sampling pulse signal is derived from one of the split AMI code signals as detected and has a pulse frequency equal to the clock pulse signal.
The decoded NRZ signals are compared with one another. When respective split AMI code signals are transmitted through the protection channel without any code error, all of the decoded NRZ signals are equal to one another.
When the protection channel is already used as an active channel or has any fault, the split AMI code signals are subjected to code errors during transmission through the protection channel, and therefore, the decoded NRZ signals are not coincident with one another.
Therefore, the condition of the protection channel is detected from the result of the comparison.
In the known protection channel monitoring system, the check signal is a pseudo random signal synchronized with the clock pulse frequency corresponding to the data transfer rate. Therefore, in order to decode each received AMI code signal to the NRZ code signal, the sampling must be carried out at each bit of the check signal. Therefore, the timing margin of each sampling point is so small that an error is apt to occur in the decoding operation.